To pretend this is the only factor the designers had in mind it absurd. To ignore tensions between the north and south in the design of the system is equally so.
> Slave states gained more power in the Electoral College from the extra 3/5 votes,
You cannot look at something with massive implications and pretend they are accidental. Unless you want to admit it's poorly designed.
> 3/5 of a vote gave less power to slave states than their preference, a full vote for each slave.
Is there a point to this statement, other than to complain the North did not fully capitulate to slave power?
-e- From Madison’s notes on the constitutional convention:
> There was one difficulty however of a serious nature attending an immediate choice by the people. The right of suffrage was much more diffusive in the Northern than the Southern States; and the latter could have no influence in the election on the score of Negroes. The substitution of electors obviated this difficulty and seemed on the whole to be liable to the fewest objections.
http://memory.loc.gov/cgi-bin/ampage?collId=llfr&fileName=00...
And no, the Electoral College wasn't something that the slave states wanted and the free states opposed. As I said, it was a compromise between big states like Virginia (slave) and Pennsylvania (free) versus small states like New Jersey (free) and Delaware (slave). Virginia (and several small but fast-growing southern states) wanted completely proportional representation (benefiting from its large slave population); Delaware and New Jersey wanted each state to be equal; the Connecticut compromise gave one chamber of Congress to each method, and in turn became the method by which the Electoral College apportions votes for the presidency.
Let me repeat:
* The Electoral College was not created or designed to protect slavery, or somehow enshrine it in the Constitution.
* It, and the composition of the two chambers of Congress, were created to give both large and small states a say in governance.
* To the extent the three fifths rule affected the Electoral College, it reduced the slave states' power versus what they wanted, which was a purely proportional system including slave populations.